1. Field of the Invention
The present invention relates to an information storage medium such as an optical disk, which causes a phase transformation between crystalline and amorphous phases in a recording layer upon irradiation by a light beam such as a laser beam, thereby recording or erasing information.
2. Description of the Related Art
As a conventional information erasable optical disk, a phase-transformation type optical disk is known. In the phase-transformation type optical disk, an irradiated portion of a recording layer is reversibly changed between two different structural states by changing the radiation conditions of the laser beam to be radiated onto the recording layer, thereby recording/erasing information.
As materials of the optical disk, semiconductors, semiconductor compounds, and intermetallic compounds such as Te, Ge, TeGe, InSe, SbSe, SbTe, and the like are known. Each material can have two states, i.e., crystalline and amorphous phases, according to the radiation conditions of a laser beam, and has different complex refractive indices N=n-ik in these states. An idea for realizing an erasable optical memory by reversibly changing the state of a laser beam irradiated portion of a recording layer between crystalline and amorphous states by thermal treatment with a laser beam is proposed by S. R. Ovshinsky, et al. (Metallurgical Transactions 2, 641 (1971)). According to this idea, the irradiated portion is selectively set in one of the crystalline and amorphous states according to the radiation condition of the laser beam, and these two states are distinguished by a difference in refractive index upon irradiation by a reproduction laser beam, thereby reproducing information. When information is to be recorded, a laser beam having a small pulse width is radiated on the recording layer at a high power capable of heating the radiated portion to a temperature exceeding its melting point so as to melt the radiated portion, and the radiated portion is rapidly cooled, thereby forming an amorphous recording pit. When the recorded information is to be erased, a laser beam having a relatively large pulse width is radiated on the recording layer at a power capable of heating the radiated portion to a temperature exceeding its crystalline temperature, and the radiated portion is gradually cooled, thus returning the recording pit to a crystalline state.
Recording/erasure of information is performed by a two-beam method using a laser beam forming a circular spot used for rapid-cooling amorphization, and an erasure laser beam forming an elliptic spot used for gradual-cooling crystallization.
However, the two-beam method complicates the optical system. In particular, since it is difficult to track an elliptic spot along the spiral track of a disk, a complicated tracking mechanism is required.
Thus, extensive studies concerning a one-beam method for performing recording and erasure by one laser beam have been carried out. In the one-beam method, in principle, an overwrite operation can be easily performed in which a laser beam radiated from a single laser is power-modulated between two power levels P.sub.E (erasure) and P.sub.W (recording) (P.sub.W &gt;P.sub.E), thereby overwriting new information while erasing recorded information. A method of performing the overwrite operation by one beam in this manner is called a one-beam overwrite method. The one-beam method can also eliminate other drawbacks of the two-beam method.
When the one-beam overwrite method is applied to a phase-transformation type recording layer, the following difficulty is involved. More specifically, whether a beam radiated portion of the recording layer is set in an amorphous or crystalline state is determined by only the power level of a radiated laser beam. Therefore, gradual-cooling crystallization cannot be performed in an information erasure mode. More specifically, crystallization must be performed within a period of time as short as that for amorphization. When information is recorded, since the power level of a recording beam is superimposed on an erasure power level, a portion around the portion on which an amorphous recording pit is to be formed is heated by a laser of an erasure power level, and is difficult to be rapidly cooled, thus disturbing formation of the amorphous recording pit.
In this manner, since the one-beam overwrite operation has difficulty in a phase-transformation type recording layer, a recording layer material which can realize the one-beam overwrite operation has not yet been found.